For many years forced air heating systems wherein a furnace has a fuel burning apparatus heating air and the air is circulated to a space in which the temperature is being controlled by a circulation fan, have used temperature responsive fan controls such as the L4064B Fan and Limit Controller shown in Instruction Sheet 68-0024-1, Rev. 1-85, of Honeywell Inc. In such a system, when a burner operates and the temperature of the furnace or plenum reaches some predetermined temperature, the air circulation fan is energized and remains energized until the operation of the heating device is terminated and the temperature of the air in the plenum drops to some predetermined lower temperature. The temperature for starting up the fan and stopping the fan has some temperature differential which is generally preset in the fan control. With such fan controls, the placement of a temperature responsive element of the fan control in the furnace plenum is quite critical to prevent the fan from operating for a longer time period than necessary and circulating cold air to the space.
Other types of fan controls make use of timers which, when the heating device is energized and the plenum air temperature increases, the fan is turned on by a temperature responsive element and the termination of the fan operation takes place after some predetermined time. Still other types of fan controls are completely time controlled in that they turn on the fan a predetermined time after the heating apparatus is energized and turn off the fan a predetermined time after the heating apparatus is turned off. Such time controlled fan controls must be tailored to the particular furnace and are not universally adapted to provide the most satisfactory operation for a furnace.
The present invention is concerned with an adaptive time control for the turn off of a furnace circulator fan. The furnace is normally turned on through a conventional room thermostat and at the same time a circulation fan is energized. Depending upon the length of the time of operation of the heating apparatus or burner, upon the termination of the burner, the circulation fan is maintained energized. If the furnace is operated for longer periods of time and a considerable amount of heat is stored in the furnace, after deenergization of the furnace, the circulation fan will be maintained for a longer period of time.
Specifically, the invention concerns a timer control which is adapted for use to control the continued operation of the circulation fan after the burner is turned off having a first timer which has a high and a low timing count fill rate and a second timer which has a high fill rate. The first timer is reset upon receiving an input signal and provides an output signal for energizing the circulation fan. Simultaneously with the operation of the circulation fan, the second timer is energized. Upon termination of the burner operation by removal of the input signal to the first timer, the first timer is energized at a high counting fill rate but, depending upon the length of time the burner has been operating, the second timer can adjust the fill rate of the first timer to provide a period of circulation fan operation dependent upon the burner ON time.